1. Field of the Invention
The present invention generally relates to bicycles, and more particularly to hydraulically-operated control systems for bicycles.
2. Description of Related Art
Bicycles are equipped with multiple gears by operating a drive chain between a cluster of larger diameter sprockets on the pedal crankshaft and a sprocket cassette mounted on the rear wheel axle. A device known as a derailleur is used to position the drive chain on a given sprocket. Existing mechanical derailleur shifting systems usually incorporate four-bar linkage mechanisms that are controlled by tension cables. A rear derailleur is located on the lower slack side of the drive chain at the rear wheel, and a front derailleur is located on the upper tight side of the chain at the sprocket cluster attached to the pedal crankshaft. The shifting control mechanism for mechanical derailleurs can be mounted on the front down tube of the bicycle frame, but most commonly is mounted on the bicycle handle bars. The shifting control mechanism typically has a small diameter detented or ratcheted pulley around which the derailleur control cable is wrapped. Each derailleur is controlled by its own shifting control mechanism.
So-called "mountain" bicycles (designed for off-road use) have become increasingly popular over the past several years, and cyclists are riding these bikes in more harsh environments. Many new innovations have been incorporated into these bicycles to increase their comfort, control, and durability. Elastomeric, air, and hydraulic suspension systems have been added to both the front and rear of mountain bicycles. New hydraulic wheel and disc braking systems have also been introduced. Unfortunately, the front and rear chain shifting systems have remained essentially unchanged, and still use mechanical cables to control the chain guiding mechanisms.
These derailleurs provide satisfactory performance in relatively clean and dry operating environments like those experienced by road bicycles. However, when mountain bikes are used in the dusty, wet, and muddy conditions found on rugged and primitive roads and trails, various contaminants work their way into the derailleur's pivots and open linkages as well as into and under tension control cables. Such contamination makes the derailleurs difficult to shift, adversely affects their precision in shifting, and also makes them wear out very quickly. The cyclist is thus required to clean, lubricate, adjust and replace mountain bike derailleur components much more frequently. Under extremely harsh operating conditions, existing mechanical derailleurs will fail, making it impossible for the cyclist to change drive ratios.
Products are available which attempt to protect the current design of derailleurs from harsh mountain biking environments. Rubber covers or boots provide some protection for the parallel linkages on the front and rear derailleur mechanisms, but these covers are not waterproof, and mud and water can still enter the mechanisms. New styles of specially coated derailleur control cables are also being offered which are designed to be less affected by mud, water, and dirt. However, dirt can still enter these control cables and cause the cables to stick or bind inside their flexible guide housings.
Alternative derailleur designs exist which do not require tension cables, such as the hydraulically-actuated, manually-controlled shift system described in U.S. Pat. No. 3,742,777. In that design, conventional derailleurs are positioned using sealed actuating bellows which extend within the front or rear derailleur under the influence of hydraulic pressure generated within similar control bellows located in handle bar controller units. While this design avoids the problems associated with contamination of tension cables and related components, it presents several other problems. The first problem relates to the limited amount of force that can be applied with this design. When a partial internal vacuum is generated within the actuating bellows by movement of the control handle to pull open the control bellows, this partial vacuum causes external atmospheric pressure to exert a force on the end face of the control bellows. However, the partial vacuum also acts upon the soft side walls of the bellows, tending to cave them inward. This effect reduces the vacuum, so the actuating bellows does not reproduce the same unit of motion as was input by the cyclist at the control bellows, especially considering that the bellows must overcome the preload within the locating detents of the guide, as well as derailleur mechanism frictional forces, and the force required to push the chain from one sprocket to the adjacent sprocket. These combined axial forces can be in excess of 20 to 30 pounds.
By using a bellows with soft enough side walls to easily collapse under atmospheric pressure, the side walls would also tend to bulge outward when the bellows were pressurized, making it more difficult for the bellows to generate displacements that were directly proportional to the motion of the input bellows in the cyclist control units.
The flexible actuating bellows could be replaced with rigid side-wall bellows to prevent side-wall cave in. If this were done, however, the external pressure required to compress the bellows would likely exceed that available from atmospheric pressure, especially when the bellows is nearly fully compressed. Furthermore, such a system could no longer be totally sealed due to inherent leakage around the rod end seals of the cylinders.
Another shortcoming in the sealed bellows system is that it has no way of compensating for the expansion and contraction of hydraulic fluid. Bicycles today are being operated in more harsh conditions and wider temperature extremes than in the past, and the expansion and contraction of a fixed amount of hydraulic fluid within a sealed bellows system would be significant. The change in volume could result in the actuating bellows being at a different length than the control bellows, which could alter the range and locations of the shifting positions in the derailleurs with respect to the control devices.
Still another concern regarding the '777 system is cavitation of the hydraulic fluid when placed in a vacuum mode. Hydraulic fluids are traditionally designed to be used in a bulk compressive mode, and not to transfer force or motion in a vacuum mode. When some hydraulic fluids are placed in a vacuum, they bubble or cavitate, again reducing the level of vacuum in the system which would also reduce any pulling effect generated by the actuating bellows. While a bellows system could be constructed to maintain a sufficient vacuum, the actuation and control units would be excessively large in light of the cyclists' demands for minimum space and minimum weight.
It would, therefore, be desirable to devise an improved hydraulic shifting system which avoids the problems associated with mechanical derailleur control devices, and further provides precise and simple control in a compact design. It would be further advantageous if the improved hydraulic shifting system could be easily adapted to automatic shift control, and used in combination with other hydraulic systems, such as brakes.